1. Field of the Invention
The present invention relates to an antenna (system) used for wireless communication, and also relates to an antenna structure and to a manufacturing method therefor for easily realizing a monopole array antenna with a high accuracy, used for high-speed data communication such as an LAN, using microwave, quasimillimeter wave, or millimeter wave band.
This application is based on Patent Applications Nos. Hei 9-119385, Hei 9-262533, and Hei 9-270858 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, antennas for terminal units used for on-site wireless LANs and the like are arranged on a desk, at a personal computer or workstation, at the upper end of a partition, or the like.
In high-speed radio communication having a transmission rate more than 10 Mbps, such as a wireless LAN, antennas with high directivity and actual gain are required. On the other hand, terminal units desirably have a function of radiating a beam in all directions (i.e., 360.degree.) in a horizontal plane, so that radio waves can always be received regardless of directional arrangement of a base station. A 19 GHz-band wireless LAN with high transmission rate of 25 Mbps and maximum throughput of 15 Mbps has been developed based on RCR STD-34 standard (refer to "19 GHz band Data Transmission Radio Equipment for Premises Radio Station", Research & Development Center for Radio Systems, March, 1993). (Also refer to "VJ25 System: 19 GHz High-speed Wireless Lan System", NTT REVIEW, Vol. 9, pp. 86-92, January, 1997.)
Regarding such a system, an analysis report is known in which specification of a terminal necessary for realizing a transmission rate of 25 Mbps with an omnidirectional antenna at a base station is analytically examined using a model based on geometrical optics. In the specification, "half-width in horizontal (or conical) plane: 30.degree., half-width in vertical plane: 30.degree., directivity gain: 15 dBi or more" are defined.
As a conventional antenna which can realize the above specifications, a three-dimensional corner reflector and a three-dimensional corner reflector provided with a dielectric material are known (refer to T. Shirato, et al., "A 19 GHz Band Wireless LAN", NTT R&D, Vol. 45, No. 8, pp. 95-104, August, 1996).
In these antennas, directivity or beam width are basically determined according to the sizes of the aperture and ground plane radius. Therefore, it is difficult to reduce the height and the diameter of the ground plane (or plate), and accordingly, weight cannot be reduced.
Another technique in which a fin is provided in order to lower (the height of) a three-dimensional corner reflector has been proposed in Japanese Patent Application, First Publication, No. Hei 9-135115. However, in this case, the fin is arranged in parallel to the ground plane; thus, the structure is complicated and processes necessary for manufacturing are increased.
On the other hand, a planar patch antenna having a smaller volume than the above three-dimensional corner reflector, for realizing reduction in size, has been proposed (refer to K. Uehara, et al., "A 20 GHz 12 Sector Antenna Using Planner Multibeam Arrays", IEICE, Proceedings of the '96 General Conference, B-107, 1996").
However, it is necessary (i) to secure sufficient aperture in the longitudinal direction so as to obtain an exact directionality in the vertical plane, and (ii) to secure sufficient aperture in the cross direction so as to obtain an exact directionality in the horizontal plane. Therefore, also in this case, it is difficult to realize an antenna having a thinner structure, and loss in a feeder circuit is increased.
Another conventional example using a horn antenna for a 6-sector wireless LAN terminal unit has also been proposed (refer to James E. Mitzlaff, "Radio Propagation and Anti-Multipath Techniques in the WIN Environment", IEEE Network Magazine, Vol. 5, No. 6, pp. 21-26, November, 1991). The size required in this case is 20 mm (longitudinal direction).times.15 mm (cross direction). Therefore, if twelve 12-sector antennas with a narrower beam are arranged using the above structure, both the necessary opening area of the aperture and the necessary number of antennas are doubled, and thus the total size becomes almost four times greater. That is, reduction in size is also difficult in this case.
As another type of antenna, an arrangement in which a monopole array antenna is arranged on a ground plane in a circumferential direction so as to cover the area with respect to the circumferential direction (see Japanese Patent Application, First Publication, No. Hei 9-36654). According to this antenna, the same characteristics as the above corner reflector antenna can be realized at one-third the height in comparison to the corner reflector antenna (refer to T. Maruyama, et al., "Design and Analysis of Small Multi-Sector Antenna for Wireless LANs Made by Monopole Yagi-Uda Array Antenna", Transactions of IEICE, B-II, No. 5, pp. 424-433, May, 1997). Here, the monopole Yagi-Uda antenna is a kind of monopole array antenna.
FIG. 12A is a perspective view showing the appearance of a conventional monopole array antenna. In this example, radiator 21, reflector 22a, and plural (here, 10) parasitic elements (or directors) 22b-22k are arranged with a predetermined space left between each two of these elements, in (the same plane of) plate 20 made of a conductive material, and connector (or connecting section) 23 is provided on the back face of the conductive plate 20. Conduction between the core wire of a coaxial cable introduced from a radio transmitting and receiving device (not shown) and radiator 21 can be established using the connector 23.
Conventionally, the monopole array antenna of such a structure is made in a manner such that pole antenna elements operating as radiator 21, reflector 22a, and parasitic elements 22b-22k are previously processed to have specific sizes, radiator 21 is disposed in hole 24 (for inserting the radiator); reflector 22a is pressed into hole 25a (for inserting the reflector); and parasitic elements 22b-22k are respectively pressed into holes 25b-25k (for inserting the parasitic elements), these holes being arranged having a predetermined space between each two of them, as shown in FIG. 12B of the corresponding partial cross sectional perspective view.
Here, the monopole array antenna is formed such that the lengths of antenna elements are arranged in order from 0.25 to 0.2 times as long as the wavelength and that no antenna length (or height) exceeds the height of the respective ordered antennas. If the transmission frequency rate is 19 GHz, the corresponding wavelength becomes 15 mm; thus, the height difference between adjacent antenna elements is defined with an order of 0.01 mm level and thus machining is very difficult. That is, in order to manufacture such a precision antenna, measurement of an accuracy using a special microscope is necessary after basic processing is completed, and readjustment is further necessary if any error is found. Therefore, in consideration of mass production, the cost required for adjustment of antenna elements is greatly increased, and consequently manufactured antennas become very expensive.